What was found
AFTER TAKEOFF AND AFTER FLAP RETRACTION WITH AUTOPILOT ON GOT A LEVEL 1 "STAB OUT OF TRIM". AUTOPILOT DISCONNECTED AND WE WERE UNABLE TO MOVE STAB TRIM EITHER WITH TRIM SWITCHES OR MANUAL HANDLES. FLEW WITH AUTOPILOT OFF AND DUMPED 100K POUNDS OF FUEL. MADE MANUAL LANDING. TRIM NEVER MOVED FROM TAKEOFF POSITION OF 8.4. REMOVED HORIZONTAL STAB CHAIN DRIVE UNIT, PER AMM 27-44-06-4. FOUND FUSE PIN SHEARED. REASON CODE: PREMATURE REMOVAL/INSTALLATION OFF MPN: AJH7337-507 POS: ONL SRL: DCA-415 ON MPN: AJH7337-507 CPN: 2710948 SRL: FX0002.